FTE's can happen for a number of reasons. You have already addressed the chamber. There are many other reasons ranging from ammunition to mechanical to a category we can call user error. Ammunition appears to be something you are addressing, but know that no manufacturer produces flawless rounds and therefore can result in FTE's, duds, etc. You may consider any of the following for mechanical: build up on the rear side (contact side) of the extractor, poor extractor geometry from the factory, a burr or other contamination in the extractor relief on the slide (I have had no less than two M&P's with this - never would have found it if i did not take them apart). The last set of issues can be things like riding the slide with your thumb - easy to do with these subs, the "limp wristing" you to which you referred and grip issues.
Good luck.
Good luck.